This invention relates to a copying apparatus capable of copying an image of an original subject in either a reduced or magnified size as well as in a size equal to that of the original.
Conventionally, in a copying apparatus, zoom lenses have been employed in the optical system for projecting an optical image onto the surface of a photoconductive drum. In this case, an effective F number is determined in accordance with a magnifying power of the lens. The amount of light of a magnified optical image is almost equal to that of one of equal size one at the center of the optical axis on the circumferential surface of a photoconductive drum as shown in FIG. 8. On the other hand, the amount of light of a reduced optical image is more than that of the equal-sized or magnified image at the center of the optical axis on the circumferential surface of the photoconductive drum.
To correct the above nonuniformity in the light amounts of the projected images, it has been proposed in Japanese Patent Provisional Publication SH060-134226 that a light shading board can be located adjacent to the lens. The light shading board can be moved toward and away from the surface of the photoconductive drum, along the optical axis, in a manner corresponding to the magnification/reduction ratio of the image to be projected by the optical system. In the copying machine employing the above-described light shading board, when a light distribution on the surface of the photoconductive drum in its axial direction is made uniform by correcting the illumination characteristic of the light directed to the original subject for equal-sized copying, the light distribution characteristic on the surface of the photoconductive drum in its axial direction at reduced size copying and at magnified size copying are as (shown in FIG. 9. That is, at reduced copying (see examples of 0.5X, 0.7X), the brightness of the longitudinal edges of the projected image field is relatively high. However, magnified size copying (see examples of 1.42X, 2.0X), the brightness of the longitudinal edges of the projected image field is relatively low.
In conventional copying machines, the magnification range is set at about from 1.24 to 0.7 of the original size. In the above magnification range, the nonuniformity in the amounts of light of the projected image influences the quality of the image negligibly and thus there causes no problem.
Recently, however, the magnification range has been broadened to extend from 2.0 to 0.5 of the original size, which makes it necessary to correct for the nonuniformity in the amount of light, especially for reduced copying.
To correct this nonuniformity in the amount of light of the projected image in the longitudinal direction of the photoconductive drum, it has been further proposed to locate a light shading board between the lens and the photoconductive drum depending upon the magnifying power of the lens, as disclosed in Japanese Patent Provisional Publication SHO57-68872.
The above copying machine, however, must be constructed as that the light shading board is movable, which makes it necessary to employ a driving device for the light shading board. Therefore, manufacture of the copying machine becomes complicated.